Method of treating boron coated steel to eliminate luders&#39; bands



F. GAROFALO METHOD OF TREATING BORON COATED ST EEL Sept. 29, 1964 To ELIMINATE: LUDERS BANDS Filed Dec. 4, 1962 l Q Q W m umfhml QQN BDN | bbw mpmfmP /NVE/VTOR FRANK GARUFALO Attorney QON kwam

United States Patent O 3,151 001 METHE GF TREATTNG I Ullhl CATED STEEL T@ ELIMTNATE LUDERS BANDS Frank Garofalo, Pitcairn, Pa., assigner to United States Steel Corporation, a corporation of New .lersey Filed Dec. 4, 1962, Ser. No. 242,128 2 Claims. (Ci. 14S-14) This invention relates to a process for treating sheet steel to reduce the tendency thereof to exhibit Liiders bands on cold working.

Diiticulty is often experienced in the cold-forming of steel sheets into various articles because of the tendency of the material to exhibit stretcher' strains or Liiders bands. Temper rolling is one method of avoiding stretcher strains temporarily but the tendency toward stretcher straining returns after a time. Another treatment for the same purpose is heating to a temperature of 13S-0 F. in a wet hydrogen atmosphere.

I have invented a novel method of eliminating or reducing the tendency of steel sheets to exhibit stretcher strains on cold deformation. Briefly stated, my method comprises coating sheet steel with boron and subjecting it to prolonged heating at a temperature of l650 F. or higher. A complete understanding of my method may be obtained from the following detailed explanation of a typical example of the preferred practice thereof. The drawings are curves depicting the results of my method.

More particularly, my method .tor eliminating from annealed low-carbon steel sheet the tendency toward formation of Lders bands or discontinuous yielding, involves coating the sheet, which may vary in thickness from 0.005 to 0.080 inch, with boron, to a thickness Varying from 0.0005 to 0.005 inch, and giving the coated sheet a long heat treatment at 1650 to 20007 F. in an inert or nonoxidizing atmosphere or vacuum.

In somewhat greater detail my improved method for reducing or eliminating discontinuous yielding and Lders bands or stretcher strains from annealed low-carbon steels including rimmed steel, may be summarized as follows:

(a) Coat steel with boron suicient to diffuse in a concentration at least 0.003% to the center of the sheet thickness. (The coating operation may be performed successfully by the flame-plating process, by boronizing, electrolytic deposition or by exposing the steel to a boride gas.)

(b) Heat steel to a temperature of from 1650 to 2000 F. in neutral atmosphere or Vacuum to prevent loss of boron by oxidation (or perform the boron deposition at these temperatures);

(c) Hold steel at temperature for from 1A: to 8 hours depending on steel thickness and temperature; and

(d) Cool steel at any convenient rate.

ln a typical example, using a rimmed steel (which shows the greatest propensity to discontinuous yielding and Lders band formation), 0.040 thick sheet specimens were coated with boron to a thickness of slightly less than 0.005 inch by the tlame plating-process. This co ating may, for example, be that performed commercially by Linde Company, Speedway, Indiana. After the coating, a 1650o F. heat treatment of four hours in a vacuum eliminated the tendency toward discontinuous yielding, for all practical purposes, and also minimized strain-aging.

The eiect of the method as described above is shown in FIGURE 1 wherein curve A, for a .specimen treated by my invention, shows no discontinuous yielding. The uncoated specimen, curve B, however, shows a fair degree of discontinuous yielding. Both specimens exhibit strain-aging but the coated sample shows appreciably less.

FIGURE 2 shows the effect on specimens treated in vacuum at 17007 E. for six hours and then air cooled. Curve A for the boron-coated sample shows not only a lack of discontinuous yielding but also it shows a very low yield strength. One other fact which is also very important is that the coated specimen exhibits a high rate of strain hardening. This is desirable in forming operations since it leads to a more uniform distribution of deformation.

The results described thus far were obtained with specimens taken directly from a large sheet and employing full sheet thickness. The total boron content for the full thickness was 0.22% by weight. Part of this boron, however, is lixed as an iron compound and is therefore not available in solid solution where it is necessary for eliminating Lders bands or stretcher strains. To deter mine the lower limits of boron necessary for eliminating Liiders bands, the middle portion of the sheet thickness which contains much less boron was tested. These results are shown in FIGURE 3 which shows no evidence of stretcher strains or discontinuous yielding in the coated specimen. The amount of boron in the center of the sheet was 0.008% by weight.

It will be apparent from the foregoing that my method is simple to carry out and eiective for the intended purpose. The product when treated is non-aging, i.e., the tendency of stretch strain does not recur.

Although I have disclosed herein the preferred practice oi my invention, I intend to cover as well any change or modiiication therein which may be made without departing from the spirit and scope of the invention.

l claim:

l. A method of treating sheet steel to inhibit stretcher straining which comprises applying a coating of boron to the steel and then heating it to a temperature of from 1650 to 2000 F. for at least a quarter of an hour.

2. A method as dened in claim l, characterized by continuing the application of boron to the sheet until a thickness of 0.0005 to 0.005 has been deposited.

References Cited in the le of this patent UNITED STATES PATENTS 2,494,267 Schlesinger et al Ian. 10, 1950 2,528,454 Schlesinger et al Oct. 31, 1950 2,726,170 Wart et al. Dec. 6, 1955 3,063,850 Mikulski Nov. 13, 1962 

1. A METHOD OF TREATING SHEET STEEL TO INHIBIT STRETCHER STRAINING WHICH COMPRISES APPLYING A COATING OF BORON 